Such antennas are suitable for local wireless networks. Classically, the slot, for example annular, is excited by electromagnetic coupling to a microstrip line according to KNORR dimensioning rules.
With such an excitation, the impedance in the electrical plane corresponding to the excitation point typically lies between 300 and 400 Ohms depending on the parameters of the substrate and the slot. Hence, this type of supply requires an impedance transformer to reduce the impedance for an adaptation at 50 Ohms or at more common impedance values. This impedance transformation, for example on a quarter wave basis, is cumbersome, generates line loss and leads to a reduction in bandwidth.
Moreover, with this type of excitation, the polarization is linear and its direction is imposed by the excitation point. It is therefore necessary to change the excitation point to modify the polarization direction.